Note: Descriptions are shown in the official language in which they were submitted.
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NESTED INTERBODY SPINAL FUSION IMPLANTS
SUBSTITUTE SHEET (RULE 26)
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BACKGROUND OF THE INVENTION
Related Application
This application claims priority to Application Serial No. 60/132,665 filed
May 5, 1999, incorporated by reference herein.
Description of the Related Art
The use of hollow threaded perforated interbody spinal fusion implants such
as taught by U.S. Patent No. 5,015,247 to Michelson ('247), incorporated by
reference herein, is now commonplace. Nevertheless, because of the structure
and
biomechanical properties of these implants, the use of such implants has not
been
available for all patients requiring spinal fusion, but rather has been
limited to a
subset of that population. While such implants have proven to be very
successful
when used correctly, such success has not been universal.
A previously identified problem as discussed in U.S. Patent No. 4,593,409 to
Michelson ('409), incorporated by reference herein, is the frequent need for
such
implants to have a reduced combined width relative to their combined height.
This
permits the height, which is usually the implant diameter, to be sufficiently
great so
as to span the height of the distracted disc space and adequately penetrate
and
engage each of the vertebral bodies adjacent that disc space, and yet have a
significantly lesser width so that when such implants are utilized in side-by-
side
pairs, the combined width is such that the paired implants do not protrude
beyond
the width of the spine. Historically, this not infrequent situation has
deprived many
patients needing spinal fusion from use of the prior art technology as
implants of the
desired height could not safely be placed within that patient's disc space
because of
the width problem. Alternatively, downsized versions of these implants were
implanted with poor results as the implants were of insufficient size to
adequately
function for their intended purpose.
As discussed in Michelson '409, implants having various vertebral bone
engaging surface projections have the advantage of enhanced stability within
the
spine as compared to an implant having a smooth surface. The use of a thread
or
thread portions has proven particularly beneficial and have been described in
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Michelson '247. As described in Michelson's co-pending application Serial Nos.
08/484,928; 08/480,904; and 08/480,908 incorporated by reference herein,
similar
devices in which opposite vertebrae engaging arcuate surfaces are in angular
relationship to each other may be useful to be fuse the vertebrae in a more
lordotic
angular relationship relative to each other. Implants of the related art are
taller near
the end adapted to be placed proximate the anterior aspect of the vertebral
bodies
than at the opposite end adapted to be placed proximate the posterior aspect
of
vertebral bodies. The related art implants are generally wedge-shaped when
viewed
from the side. The wedged configuration causes the implant to be less stable
within
the spine than if it were non-wedged. Further limiting the stability of these
implants,
compromising the surface area available for contact and fusion, and limiting
the
volume of osteogenic material containable within the implants is a result of
the fact
that these implants have generally been relatively flat across their trailing
ends so as
to be rotationally symmetrical about their mid-longitudinal axes. The anterior
aspects of the vertebral bodies are generally curved from side-to-side. As a
result,
related art implants needed to be rather deeply inset into the disc space and
away
from the anterior aspects of the vertebral bodies so as to prevent the
implants from
protruding from the disc space at their lateral wall and trailing end
junctions,
wheresuch a protrusion of the implant could place vital structures adjacent
the spine
at risk.
There is, therefore, a need for further improvement in the design of such
interbody spinal fusion implants so as to firstly extend their range of
usefulness, and
secondly to further increase the rate of success when such implants are used.
SUMMARY OF THE INVENTION
In accordance with the present invention, as embodied and broadly described
herein, there are provided interbody spinal fusion implants that are threaded
at least
in part and require an element of rotation for insertion across a disc space
between
two adjacent vertebral bodies of a spine. The implants of the present
invention are
configured to be positioned in close proximity to each other such that the
combined
width of the implants is less than the combined height of the implants. The
implants
preferably have a leading end, a trailing end opposite the leading end, and a
mid-
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longitudinal axis and length therebetween. The implants preferably have
opposite
arcuate portions adapted for placement toward and at least in part within the
adjacent vertebral bodies and have a distance therebetween defining an implant
height greater than the normal height of the disc space to be fused. Each of
the
opposite arcuate portions preferably has at least one opening in communication
with
each other for permitting for the growth of bone from vertebral body to
adjacent
vertebral body through the implant. Preferably, at least a portion of a thread
is
formed on the exterior of each of the opposite arcuate portions for penetrably
engaging the adjacent vertebral bodies and to facilitate securing the implant
into the
spine by at least in part rotating the implant about its mid-longitudinal
axis. At least a
first one of the implants preferably has a lateral side wall and a medial side
wall with
a distance therebetween defining an implant width transverse to the implant
height.
The width of the first implant is less than its height along at least a
portion of its
length. The medial side wall of the first implant is preferably configured to
be
positioned in close proximity to at least a second spinal implant such that
the
combined width of the first and second implants is less than the combined
height of
those implants.
The present invention provides for improved interbody spinal fusion implants
for placement within the spine in longitudinal side-by-side nested pairs. As
used
herein, the terms "nesting or nested" refer to the placement of at least two
implants
in side-by-side relationship and close proximity to each other. In a preferred
embodiment, the present invention teaches the nesting together of a pair of
tapered
root threaded spinal fusion implants, such that the nested implant pair has a
combined reduced width relative to the combined height of the individual
implants.
As used herein, the terms "tapered root" refers to an implant having an outer
diameter as measured at the peaks of the bone penetrating protrusions, such as
threads, and a root diameter, wherein the root diameter tapers from one end to
the
other end of the implant.
An embodiment of the present invention includes an interbody spinal fusion
implant adapted to receive along its length a second circumferentially
threaded
complimentary interbody spinal fusion implant, such that the second implant
nests
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within the circumference of the first implant. The nested longitudinal side-by-
side pair
has a combined width less than the implants' combined maximum diameters, which
maximum diameters generally define the over-all implant heights. In a further
embodiment, the implants of the present invention are angled toward each other
such that the combined width at the leading ends is further lessened.
The present invention implants have opposite arcuate portions and preferably
are rotated into place. In an embodiment of the present invention, the
implants may
be generally cylindrical and have a thread or thread portions. In a preferred
embodiment, the root diameter of the implant is generally conical or a portion
of
cone in that the opposite arcuate surfaces for contacting the vertebrae
adjacent the
disc space are in angular relationship to each other generally over the length
of the
implants. In a preferred embodiment of a tapered root implant, the outer
diameter of
the implant as measured at the thread peaks remains relatively constant over
the
length of most of the implant. As the root diameter of the implant tapers
down, the
thread height increases such that the outer diameter of the implant as
measured at
the thread peaks remains relatively constant.
The present invention has at least one of a pair of implants having at least
one side adapted to receive within the over-all circumference of the outer
diameter
of the implant the side of a second implant. In a preferred embodiment, the
receiving implant has both leading and trailing support walls, and while
preferable,
but not requisite, these walls may provide structural support and nevertheless
be
highly perforated to allow for vascular access and the growth of bone through
the
implant. In a preferred embodiment of a second implant to be received within
the
first implant, the leading end support structure further comprises a cap,
which cap
need not, but may be threaded, and which cap need not be, but preferably is
perforated.
In a preferred embodiment of the present invention, the trailing ends of the
implants are rotationally asymmetrical about the mid-longitudinal axis such
that they
may be inserted in nested fashion and in proper rotational alignment relative
to each
other and to the vertebral bodies, with the result that the implants will have
a length
along the lateral aspect from leading to trailing end less than the length
along the
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medial wall from leading end to trailing end. Preferably, the implants of the
present
invention are structurally adapted, such that when properly inserted, the
length of the
lateral side wall as measured from the leading end to the trailing end is of a
lesser
length than the length of the implant along its mid-longitudinal axis so as to
prevent
the protrusion of the lateral side wall and trailing end junction beyond the
circumferentially curved profile of the vertebral bodies.
While the present invention does not require it, in a preferred embodiment the
implants are adapted to receive through their trailing ends opposed bone
screws and
to transmit at least threaded portions of those screws through the opposite
vertebrae
engaging arcuate portions so as to allow those bone screws to engage at least
one
each into each of the vertebral bodies adjacent a disc space into which the
implant
are implanted.
Each of the embodiments of the implants of the present invention may also
include one or more of anatomically contoured trailing ends, tapered minor
diameters, opposed bone engaging screws, and locks for locking the opposed
bone
engaging screws into place. In a preferred embodiment, the implants of the
present
invention are configured to receive bone screw locks to lock the opposed bone
screws to the implants. The bone screws are preferably lag screws and the
locks,
while preventing the backing out of the lag screws from the implant, may
either be
rigidly fixed or allow for continuing angular motion of the lag screws
relative to the
implants.
The present invention also is directed to an improved method for inserting
such implants.
The accompanying drawings, which are incorporated in and constitute a part
of this specification, are by way of example only and not limitation, and
illustrate
several embodiments of the invention, which together with the description,
serve to
explain the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevational view of the lateral aspect of the human spine
with
implants of an embodiment of the present invention implanted therein.
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FIG. 2 is a front perspective view of two adjacent lumbar vertebrae and two
implants of an embodiment of the present invention implanted across the disc
space.
FIG. 3A is a top plan view of the implants of FIG. 2 and a cross section
through a vertebra along line 3A--3A of FIG. 2.
FIG. 3B is a top plan view of another embodiment of the implants of FIG. 3A
and a cross section through a vertebra along line 3A--3A of FIG: 2.
FIG. 3C is a top plan view of another embodiment of the implants of FIG. 3A
and a cross section through a vertebra along line 3A--3A of FIG. 2.
FIG. 4A is a top plan view of the implants of FIG. 3A.
FIG. 4B is an end view of another embodiment of an implant of the present
invention with two implants shown in hidden line.
FIG. 5 is a side perspective view of an embodiment of an implant of the
present invention.
FIG. 6 is a side perspective view of another embodiment of an implant of the
present invention.
FIG. 7 is a side elevational view of the implant of FIG. 5.
FIG. 8 is a leading end view of the implant of FIG. 6.
FIG. 9 is a front elevational view of two adjacent vertebrae in a lumbar spine
with the implants of one embodiment of the present invention implanted across
the
disc space therebetween.
FIG. 10 is a cross sectional side elevational view of the lateral aspect of
two
adjacent lumbar vertebrae illustrating the bore created by a drill.
FIG. 11 is a cross sectional side elevational view of the lateral aspect of
two
adjacent vertebrae and an embodiment of an implant of the present invention
inserted therebetween.
FIG. 12 is a cross sectional side elevational view of the adjacent vertebrae
and implant of FIG. 11 with a screw driver and a bone engaging screw being
installed.
FIG. 13 is a side elevational cross sectional view of the adjacent vertebrae
and the implant of FIG. 11 with a bone engaging screw in the installed
position.
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FIG. 14 is a side elevational cross sectional view of the adjacent vertebrae
and implant of FIG. 11 with two bone engaging screws in the installed
positioned.
DETAILED DESCRIPTION OF THE DRAWINGS
The following description is intended to be representative only and not
limiting
and many variations can be anticipated according to these teachings, which are
included within the scope of the present invention. Reference will now be made
in
detail to the preferred embodiments of this invention, examples of which are
illustrated in the accompanying drawings.
Figure 1 is a side elevational view of the lateral aspect of a human lumbar
spine S having vertebral bodies V and discs D interposed therebetween. The
trailing
end of an implant 100 can be seen located in the disc space between the fourth
and
fifth lumbar vertebrae L4 and LS wherein disc space D has been distracted more
anteriorly than posteriorly.
Figure 2 is a front perspective view of adjacent vertebrae within a lumbar
spine designated as V~ and V2. Interposed therebetween is a disc D, with
paired
implants 100 and 200 in nested side-by-side opposition inserted in the disc
space.
Implants 100 and 200 have heights greater than the height of the restored disc
space into which they are implanted so as to expand the height of that disc
space
and to further penetrably engage into the bone of each of vertebral bodies V~,
V2. In
a preferred embodiment, the circumference of the trailing end of implant 100
preferably extends within the circumference of implant 200 when implants 100,
200
are in nested, longitudinal side-by-side placement.
As shown in Figures 3A, 4A, and 5-8, implants 100, 200 preferably each have
a leading end 108, 208 for insertion first into the disc space; a trailing end
104, 204,
opposite leading ends 108, 208, respectively; and a mid-longitudinal axis and
length
therebetween. Implants 100, 200 preferably each have upper and lower opposite
arcuate portions 134, 134', and 234, 234' adapted for placement toward and at
least
in part within adjacent vertebral bodies V~, V2 and have a distance
therebetween
defining an implant height greater than the normal height of the disc space to
be
fused.
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Preferably, opposite arcuate portions 134, 134' and 234, 234' each have at
least one opening 120, 220 communicating with one another for permitting the
growth of bone from vertebral body to adjacent vertebral body through the
implant.
Preferably, at least a portion of a bone penetrating protrusion, such as
thread 116,
216 is formed on the exterior of each of the opposite arcuate portions 134,
234,
respectively, for penetrably engaging the adjacent vertebral bodies and to
facilitate
securing the implant into the spine by at least in part rotating the implant
about its
mid-longitudinal axis.
Figures 5-8 show that in a preferred embodiment, implants 100, 200 have a
body which is generally frusto-conical, increasing in diameter from leading
end 108,
208 to trailing end 104, 204 and preferably has a helical thread 116, 216
about its
circumference. Thread 116, 216 can have a generally constant outside diameter
and can progress from a sharp pointed profile at the thread portion proximate
leading end 108, 208 to a thicker and more squared profile toward trailing end
104,
204. Thread 116, 216 may be interrupted as shown in Figures 5 and 6. These
specifics are shown by way of example only and not limitation. It is
appreciated that
implants 100, 200 may have any type of thread or threads useful for the
intended
purpose without departing from the present invention. Further, the implants of
the
present invention need not be frusto-conical as shown, and could be shaped
much
like a cylinder cut in half transversely through its mid-longitudinal axis
with the upper
and lower halves of the implant in angular relationship to each other.
In a preferred embodiment; the root diameter of the implants of the present
invention tapers from its trailing end for placement anteriorly to its leading
end for
placement posterior within a disc space. This configuration is particularly
desirable
for providing for the proper lordotic relationship between the adjacent
vertebral
bodies, such that those vertebral bodies are distanced apart greater
anteriorly than
posteriorly. In a further preferred embodiment of these lordotic implants, the
thread
has a generally constant outside diameter such that in combination with the
tapered
root the actual thread height increases from the trailing end to the leading
end
progressively. At the leading end the thread may have a lesser height for
facilitating
the implant insertion without departing from this teaching.
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Figures 5 and 7 show a preferred embodiment of implant 200. Lateral side
wall 228 and medial side wall 232 have a distance therebetween defining an
implant
width transverse to the implant height. The width of implant 200 is less than
its
height along at least a portion of its length. Medial side wall 232 is
preferably
configured to be positioned in close proximity to at least implant 100 such
that the
combined width of implants 100, 200 is less than the combined height of those
implants.
Implant 200 is similar to implant 100, but differs from implant 100 in that
while
the lateral sides 128 and 228 of implants 100 and 200, respectively, are the
same
and in this example convex, the medial side 232 of implant 200 has been
relieved so
as to allow for the convex medial side 132 of implant 100 to protrude therein.
Alternatively, medial side of implant 200 can be relieved, in part absent,
and/or
concave.
Implant 200 also has at medial side 232 a convexity as shown by the contour
of trailing support wall 260. In a preferred embodiment, leading support wall
256
may similarly be concave. And further a portion of the medial side wall 232 is
absent
so as to allow for the protrusion of implant 100 therein.
As shown in Figure 4B, in another embodiment of the present invention, an
implant 300 may have a lateral side wall 328 and an opposite medial side wall
332.
Both lateral side wall 328 and medial side wall 332 can be configured to be
positioned in close proximity to two implants 100, one on each side of implant
300.
The combined width of implant 300 and two implants 100 is less than the
combined
height of those implants.
Without departing from the scope of the present invention, the medial side of
the implant need not be present as a single large opening 252 and could have a
wall
or support portion to it. In a preferred embodiment as shown, the large
opening 252
allows for the easy packing of the implant with such osteogenic material as
bone or
a carrier containing bone morphogenetic protein or genetic material coding for
the
production of bone. A further advantage of the openness of this area is that
it allows
for further collateral vascularization to support bone growth from implant to
implant.
Again, while the present invention is not limited to a medial opening as
shown,
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implant wall edge 268 can be continuous longitudinally to provide for strength
in this
critical area and if desired can be sharpened as can interrupted thread bases
272 so
as to further facilitate the self-tapping nature of these implants.
In a preferred embodiment of the present invention, leading ends 108, 208
and trailing ends 104, 204 are also perforated. While leading end 208 could be
more or less open, in a preferred embodiment there is a structural support
portion
256 perforated by openings herein shown as 276. Trailing ends 104, 204
preferably
have a plurality of openings 144, 244 therethrough to allow for vascular
access into
implants 100, 200 and the possibility of bone growth therethrough. Openings
144,
244 may also communicate with further openings such as 148, 148', 248, 248' to
further increase the porosity of the trailing end of the implant. Implant 200
preferably
is both hollow and highly perforated. Alternatively, the implants of the
present
invention can comprise a porous type of material such as a cancellously
structured
tantalum.
As can be appreciated from Figures 3A-4A, implant 100 is preferably
structurally configured so as to cooperatively interdigitate into the maximum
circumference of implant 200 along its length. When implant 100 is inserted
within
the circumference of implant 200 as shown in Figures 3A-4A, the combined width
of
implants 100 and 200 is substantially less than their combined heights, which
heights in this case correspond to the maximum circumference of each of the
implants which in this example is the same.
While the implants of the present invention can have any of a variety of
configurations at their trailing ends, trailing end 104 of implant 100, and
trailing end
204 of implant 200 are preferably contoured to sit on the anterior rim AR of
vertebral
body V~. As shown in Figure 3A, trailing ends 104, 204 are asymmetrical about
the
mid-longitudinal axis of the implants 100, 200, respectively. In the final
installed
position of the implants, trailing ends 104, 204 preferably and generally
conform to at
least a portion of the anatomic curvature of the anterior rim AR of vertebral
bodies
V
As shown in Figure 3B, alternatively, trailing end 104' of implant 100' may be
symmetrical about the mid-longitudinal axis of implant 100' and trailing end
204' of
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implant 200' may be asymmetrical about the mid-longitudinal axis of implant
200'.
Trailing ends 104', 204' are preferably contoured to sit on the anterior rim
AR of
vertebral body V2.
As shown in Figure 3C, alternatively, trailing end 104" of implant 100" and
trailing end 204" may be symmetrical about the mid-longitudinal axes of
implants
100", 200", respectively. Trailing ends 104", 204" are preferably contoured to
sit on
the anterior rim AR of vertebral body V2.
As best shown in Figure 8, leading end 108 of implant 100 comprises an
externally threaded cap 112 for threading into the internally threaded opening
of
leading end 108. Cap 112 is rotatable in either direction for opening and
closing of
implant 100 by cap 112 with hex opening 172 which can be manipulated with a
hex
driver. The specifics of cap 112, such as the bone holes 176, are shown by way
of
example only and not limitation. Cap 112 is useful for allowing for the
compressive
loading of the implant with osteogenic materials such as bone, demineralized
bone
matrix, carriers such as collagen or any other material useful as a carrier
for the
delivery of bone producing materials such as morphogenetic proteins,
mineralizing
proteins, or chemical compounds or genetic material coding for the production
of
bone. The cap itself may be more or less perforated and the opening themselves
can be either larger or smaller, less or more, as desired to provide access to
the
interior of the implant, bone growth therethrough, and to the extent desired
acts so
as to contain the osteogenic material generally within the implant and from
grossly
expelling out of the leading end. The cap may be used as a structural member
adding strength to the implant overall. Depending on the specific qualities
desired,
the cap can be made of any material appropriate for its purpose. Such
materials
would include various plastics, including polyethylene, and may include
bioresorbable plastics as well. Such materials may include cortical bone,
ceramic,
or any surgical quality metal suitable for the intended purpose and including
by way
of example only and not limitation, surgical grade titanium and its alloys.
Figure 4A is a top view of nested implants 100, 200 of Figure 3A. As shown
in Figure 4A, the implants of an embodiment of the present invention are
threaded
on their exterior surface. While the embodiments shown have bone penetrating
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projections in the form of a helical thread, the present invention allows for
a
continuous thread, or an implant having thread portions such as would be
useful for
an implant to be inserted by linear advancement and then requiring an element
of
rotation, for example 90 degrees, to set the thread portions into the bone of
the
adjacent vertebral bodies.
As an alternative, the bone penetrating projecting segments need not be
portions of a thread, but rather could be concentric projections generally,
but not
necessarily, oriented perpendicular to the long axis of the implants. For
example,
the projection may be a ridge or a fin adapted to penetrably engage the bone
of the
adjacent vertebral bodies.
As shown in Figure 5, thread 116 of implant 100 may have a generally
constant outer diameter. Inasmuch as the body of implant 100 is generally
conical
such that it tapers from the larger trailing end 104 to the smaller leading
end 108, the
height of thread 116 relative to body 102 increases from trailing end 104 to
leading
end 108. Thus, while the outer diameter of the threads remains generally
constant,
the height of the thread increases from trailing end 104 to leading end 108.
This is
similarly true for implant 200.
In a preferred embodiment of implant 100, 200 the start of the external thread
about the perimeter of the implant is precisely indexed such that if the
surgeon
knows the depth of the bore created, he may select an implant of the desired
length
being less than or equal to the depth of the bore created and by starting the
insertion
of the implant in a preferred rotational alignment such as the desired final
rotational
alignment the implant when threaded in fully will come to rest such that
trailing end
104, 204 will be correctly rotationally aligned so that screw receiving holes
236, 236'
will be oriented correctly towards the adjacent vertebral bodies while the
profile of
trailing ends 104, 204 will correspond to the contour of the anterior
vertebral body.
By way of example, for a bore measured to receive a 30 millimeter maximum
length implant having a pitch of three millimeters as an example, the start of
the
thread at the implant leading end could be indexed such that the implant could
be
introduced rotationally oriented exactly as desired for the final positioning.
Then, by
making ten complete revolutions of three millimeters each the implant would
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assuredly come to rest with trailing wall 204 appropriately oriented and
either be
flush with the anterior vertebral cortices, or minimally counter-sunk to
exactly the
extent to which the surgeon caused the implant to enter the bore prior to
initiating
rotation. As previously mentioned, trailing end 204 of implant 200 could be
rotationally asymmetrical, but nevertheless be symmetrical from side-to-side,
such
that each of the sides of the implant would be less protuberant posteriorly
than a
point along the mid-longitudinal axis such that the implant could be correctly
inserted
in increments of less than or equal to 180 degrees of rotation.
While a preferred embodiment of the present invention is directed to a
tapered root implant having a constant outer diameter thread, the present
invention
is not so limited. The present invention also would include implants having
bodies
that are more or less cylindrical. The present invention also would include
other
thread designs including a thread having a constant height relative to the
body and
with or without a constant outer diameter depending on the shape of the body.
As shown in Figure 4A, extending through both upper and lower surfaces
134, 234 and located approximate trailing ends 104, 204 of implants 100, 200
are
openings 124, 124', and 224, 224' respectively, for transmitting the threaded
shafts
of bone screws for engaging the adjacent vertebral bodies. While not so
limited, a
preferred embodiment of the present invention is adapted to receive a
cancellous lag
screw, having a head portion incapable of passing out of the implant so as to
be
contained therein. It is further preferred that the implant of the present
invention are
adapted to receive a lock for locking the opposite vertebrae engaging bone
screws
to the implants. Still further preferred is that the implants of the present
invention are
adapted to receive the lag screws and screw locks so as to allow the locks to
function to prevent the backing out of the screws while, nevertheless allowing
for
some angular motion of the screws relative to the implants.
Figure 9 shows a front view of an embodiment of the present invention with
implants 100, 200 properly implanted across the disc space between adjacent
vertebral bodies V~ and V2. Openings 144, 244 of implants 100, 200,
respectively,
allow for vascular access through trailing ends 104, 204 of implants 100, 200
and for
bone growth therethrough. Trailing ends 104, 204 have common openings 126, 226
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and situated essentially therein, are threaded openings 148, 248 for receiving
an
implant driver. The implant driver has a distal end for a complimentary fit
within
common openings 126, 226 and therethrough a rotatable threaded member for
threading into openings 148, 248, respectively. Openings 126,226 also are
adapted
to receive a screw device to link the implant to other implants, to a staple,
or to
receive a locking screw to lock bone engaging screws to the implant as
disclosed in
Michelson U.S. Patent application Serial No. 08/926,334 incorporated herein by
reference. Common opening 126, 226 also may have therein opposed and
divergently angled openings 136, 236 and 136', 236' adapted to receive opposed
vertebral bone engaging screws. Bone screw receiving openings 136, 236 and
136',
236' preferably may have circumferentially around them retaining seats 140,
240
and 140', 240' adapted to receive and to block the passage of the heads of
screws
to be inserted therethrough. Retaining seats 140,240, and 140', 240' may also
be
flanged.
As shown in the preferred embodiment of the present invention, trailing ends
104 and 204 of implants 100 and 200, respectively, preferably are rotationally
asymmetrical about the longitudinal axes of the implants such that the
designated
medial side of each of the implants has a length greater than the lateral
sides of the
same implants. Trailing ends 104, 204 preferably are structured to have a
lesser
length along their lateral sides than through the mid-longitudinal axis and
are
preferably contoured so as to sit on the anterior rims of the vertebral bodies
without
protruding dangerously therefrom as set forth in pending Michelson application
Serial No. 09/263,266 incorporated herein by reference. In another embodiment
of
the present invention, the trailing ends of the implants can have a maximum
length
along the mid-longitudinal axis greater than the length along either of the
medial and
lateral sidewalls so that the bone screw receiving holes can be oriented
towards the
adjacent vertebral bodies in half rotation increments rather than requiring a
full
rotation. While for implant 100 this would require no other modification than
as
described for the trailing end, in regard to implant 200 each of the lateral
and medial
side walls would have to be relieved, as shown in Figure 4B for example, to
allow for
CA 02363562 2001-08-14
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the receipt of the perimeter of implant 100 within the maximum perimeter of
implant
200.
Figures 10-14 show a series of steps useful for discussing a method of use of
the present invention implants. Methods for inserting spinal implants are
discussed
in part in issued and pending patent applications to Michelson 5,593,409,
5,741,253,
5,484,437, 08/396,414, and 08/480,904, incorporated by reference herein. The
disc
space to be used is preferably, but not necessarily, distracted to optimal
height and
the vertebral bodies preferably, but not necessarily, properly aligned. A pair
of
overlapping bores are then formed across the disc space with a bone removal
device such as a drill having a diameter greater than the height of a
distracted disc
space such that arc-shaped portions of bone are removed from each of the
vertebral
bodies adjacent the disc space to be fused. The overlapping bores are
generally
oriented from anterior to posterior and preferably stop short of the spinal
canal.
A bone removal device such as a drill or mill that may be conical can be
utilized to complement the tapered configuration of the implant body. As shown
in
Figure 10, however, in a preferred method a generally cylindrical drill DR or
end mill
is utilized to create a generally cylindrical bore for receiving the implants.
When a
pair of generally cylindrical overlapping bores, preferably but not
necessarily, having
a diameter generally corresponding to that of the root diameter of the implant
proximate the leading end are formed as per Figure 3A, the implants will come
to be
positioned such that the combined width of the implants at their leading ends
will be
less than the combined width of implants at their trailing end. That is, the
implants
will be angled in towards each other from anterior to posterior. This has the
further
benefit of swinging the junction of the lateral side walls and trailing ends
further
inward and away from escaping the anterior vertebral cortex, thereby avoiding
protrusion of the lateral side wall to trailing end junctions and allowing for
the
installation of larger and longer implants than might otherwise be possible.
As has been taught by Michelson in the above identified applications and
patents incorporated by reference herein, the disc space may be distracted in
any
number of ways and held distracted during the bore formation portion of the
procedure. Some of the preferred ways include the use of long distractors,
short
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WO 00/66044 PCT/US00/12363
distractors, and extended outer sleeves having distractor members for
placement
within the disc space and between the adjacent vertebral bodies as described
by
Michelson in the above described applications and patents incorporated by
reference herein. Other distractors such as those which attach to the
vertebral
bodies as by pins or screws might also be useful for the present intended
purpose.
While surgery may be performed through a single bore first, in a preferred
embodiment both bores are created in overlapping fashion prior to the
insertion of
the first implant which in this example is implant 200. Implant 200 is affixed
to an
implant driver which driver preferably engages the implant at trailing wall
204 by
interdigitating with implant 200 and further binding to implant 200 by a
thread such
that it is possible both to rotate implant 200 in either direction and to push
or pull
simultaneously. While that may be achieved by having a driver which
interdigitates
with any of the openings into or through rear wall 204 and having a rotatable
portion
for threading into threaded opening 248 the present invention is not so
limited and
may include any driver useful for the intended purpose.
After implant 200 is fully seated with medial side wall 228 oriented
immediately toward the disc space, a complementary implant 100 is inserted by
allowing it to rotate within the maximum circumference of implant 200. Pre-
tapping
the bores formed across the disc space prior to the insertion of the implants
does not
deviate from the present teaching. In a preferred embodiment, pre-tapping is
not
required as certain preferred embodiments of the present implants are tapered
from
their trailing to their leading ends and the leading ends have particularly
significant
thread heights making their ability to thread themselves into the bone
particularly
effective.
Figures 11-14, show openings at the trailing end of the implant for receiving
opposed screws that may be oriented from the implant into each of the adjacent
vertebral bodies. These screws enter the implant through the trailing end and
the
threaded shafts of the screws pass through openings in the opposite upper and
lower vertebral body engaging surfaces of the implants. Shown in Figure 11 is
a cut
away through implant 100 of Figure 9. This is a cross section through the mid-
longitudinal axis of implant 100 and the adjacent vertebral bodies. Figure 12
shows
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a screw driver 450 driving a bone screw 500 through common opening 126, bone
screw receiving hole 136' and out opening 124' through lower vertebrae
engaging
surtace 134' into adjacent vertebral body V2. The present invention includes
the use
of any bone screws for this described purpose. In preferred embodiments,
structure
is provided to block the bone screws from disengaging from the implant or
backing
out. The screws may be rigidly locked to the implant or may be prevented from
backing out in a manner that still allows for some relative motion between the
screws
and the implant. The latter may be beneficial for anticipating and allowing
for some
settling of the vertebral bodies towards the disc space.
In use, as shown in Figure 12, the driver 450 is assembled to the screw 500
thereby compressing the head portion of the screw. The screw is then
introduced
through the trailing end of the implant and directed into the body of one of
the
adjacent vertebrae passing out of an opening adapted for that purpose in one
of the
opposite vertebrae engaging surfaces of the implant. The head of the screw 500
is
too large to pass through the opening in the implant, and yet is free to spin
against
the implant itself making it possible to lag the screw, or that is to draw the
body of
the vertebra to the implant and to generate compressive load between the
implant
and the vertebral body.
As shown in Figure 13, when the screw has been fully seated and the driver
removed, the head of the screw is free to reexpand, thereby locking it to the
implant.
As mentioned, the present invention includes the use of any opposed vertebrae
engaging bone screws, such that at least one each of said screws binds the
implant
to each of the adjacent vertebral bodies. In a preferred embodiment, the
screws are
prevented from backing out of the implant. Screws may be locked directly to
the
implant such that they are rigidly attached thereto, or may be capable of some
movement relative to the implant so as to allow for variation in screw
positioning
and/or settling of the vertebrae, and yet be prevented from backing out.
While the present invention is shown in a preferred embodiment as both
highly perforate and substantially hollow, the implant could comprise a
generally
porous or cancellous material allowing for the growth of bone in continuity
from
vertebrae to vertebrae through the implant. The present invention implants can
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w0 00/66044 PCT/IJS00/12363
comprise of any material that is biocompatible and structurally suitable for
construction of these interbody spinal fusion implants and consistent with the
growth
of bone from vertebral body to vertebral body through the implants. To that
end,
materials which would be satisfactory might include implant quality metals
such as
surgical quality titanium and its alloys, cobalt chrome or other metals useful
for this
purpose, cortical bone and in particular human cortical bone such as that
which
might be obtained from one of the tubular long bones of a human body,
ceramics,
plastics and composite materials including those incorporating carbon fibre;
and
such implants may further comprise, contain, be treated with or coated with
osteogenic materials other than bone for the purpose of achieving spinal
fusion.
Such materials would include but not be limited to bone morphogenetic protein,
ossification inducing proteins, and genes coding for the production of bone.
Further,
the implants may include at least in part or wholly materials bioabsorbable by
the
human body, which by way of example only may include plastics selected from
the
lactide, lactone family, polylactide, polylactone family, glycolic acid
derivatives and
so forth.
The implants of the present invention may be coated with, comprised of, be
used in combination with, or have a hollow for containing bone growth
promoting
materials, including but not limited to, bone morphogenetic proteins,
hydroxyapatite,
and genes coding for the production of bone. The implants of the present
invention
can be formed of a material that intrinsically participates in the growth of
bone from
one of adjacent vertebral bodies V to the other of adjacent vertebral bodies
V.
While the specific preferred embodiments the implants of the present
invention have been described, again the present invention is not so limited.
The
present invention includes any interbody spinal fusion implants embodying the
present teachings including implants having opposite surfaces with a thread,
portion
of a thread, or generally concentric fins used to penetrably engage the
substance of
the vertebral bodies when the implants are rotated about their longitudinal
axes
approximately 90 degrees or more.
The implants of the present invention are preferably hollow with a plurality
of
openings through the various surfaces of the implant in communication with the
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implant hollow. The present invention includes either fewer, or more openings
so
long as each of the opposite vertebrae engaging surfaces of the implant have
at
least one opening in communication with the other so as to allow for the
growth of
bone in continuity from vertebral body to adjacent vertebral body through the
implant.
There is disclosed in the above description and the drawings implants, which
fully
and effectively accomplish the objectives of this invention. However, it will
be
apparent that variations and modifications of the disclosed embodiments may be
made without departing from the principles of the invention or the scope of
the
appended claims.
